Much of today's high-speed data communication traffic is carried out in two directions simultaneously; that is, the traffic is full duplex. For data speeds below about 2400 bits per second the voiceband extending from approximately 300 to 3000 Hz can be divided in half to allow dedication of each half to a particular transmission direction on an equivalent four-wire transmission line basis. For data speeds above 2400 bits per second two physically separate two-wire transmission lines of voice bandwidth are currently required. If two-way high-speed simultaneous communication between two points could be accomplished at full bandwidth on a single two-wire channel, significant cost savings would become possible. Furthermore, a capability of simultaneous two-way digital data communication on the public switched direct-distance-dialing (DDD) network would be valuable in cases where the data customer could benefit from zero "turn-around" time. Under present practices reversing the direction of transmission on a half-duplex DDD transmission channel requires time to disable built-in echo suppressors.
Heretofore, full-duplex data transmission on a two-wire transmission channel has been implemented by utilizing separate segments of the available transmission band, each dedicated to a preassigned transmission direction. This split band method, while feasible and fairly simple to implement, has these disadvantages:
1. The sum of the data rates for the two transmission directions is less than or at most equal to the otherwise attainable one-way data rate for the given channel operating in the half-duplex mode; and PA1 2. Each transmitter uses a different frequency band, depending on whether it is at the originating or terminating end of a communication channel. This circumstance can result in equipment duplication and elaborate transmission protocols, i.e., start-up routines.
Many long-haul toll telephone facilities include echo suppressors which are designed to suppress reverse traffic when forward traffic has seized the facility. Whenever the direction of traffic is to be reversed, one set of echo suppressors must be disabled and another set activated. Thus, simultaneous two-way traffic is precluded without special arrangements for disabling all echo suppressors. Such arrangements are routinely included in many full duplex voice grade modems. With speech traffic it is not usually contemplated that both parties will be speaking simultaneously.
For simultaneous two-way transmission within the same frequency band it is mandatory to separate the local transmitter signal from the usually weak signal received from the remote site. Hybrid networks or bridge circuits, realizable with or without transformers, are standard, well known arrangements for achieving this separation. In such circuits, a terminating impedance equal to the impedance of the two-wire line must be used for perfect separation. Due to the complex and frequency dependent nature of this impedance only a very approximate compensation is possible in practice with passive circuits. Direct current leakage across the hybrid and delayed echoes caused by signals reflected from more distant impedance mismatches will cause transmitted signal components to interfere with the received distant signal.
Adaptive echo cancellers implemented by transversal filters have been proposed for analog facilities by, for example, J. L. Kelly, Jr., and B. F. Logan, Jr., in U.S. Pat. No. 3,500,000 issued Mar. 10, 1970. In the latter echo canceller a portion of the analog signal incoming to a hybrid junction on the four-wire side is passed through a transversal filter with adjustable tap-gain controls to synthesize a cancellation signal for subtraction from the signal outgoing from the hybrid junction. The resultant outgoing signal is clipped and correlated with the sequence of samples of the incoming signal appearing at the taps of the transversal filter to form control signals for the tap gains or weighting coefficients of the transversal filter.
A similar arrangement is described by F. K. Becker and H. R. Rudin in the Bell System Technical Journal (Vol. 45, 1966, pp. 1847-1850), in a paper entitled "Application of Automatic Transversal Filters to the Problem of Echo Suppression." Results achieved with a practical realization of the transversal filter as an adaptive echo suppressor are reported by V. G. Koll and S. B. Weinstein in I.E.E.E. Transaction on Communications, (Vol. COM-21, No. 2, 1973, pp. 143-147) in a paper entitled, "Simultaneous Two-Way Data Transmission Over a Two-Wire Circuit."
The aforementioned echo cancellers employing transversal structures require tap spacings at the reciprocal of twice the highest frequency present in the message-signal to be transmitted, principally because the signal to be transmitted is assumed to be analog in nature. In the typical voice bandwidth of 4000 Hz eight taps are required for each millisecond of anticipated echo delay. Such intervals are known as Nyquist intervals and relate to the transmitted signal as the reciprocal of twice the highest frequency present.
In the copending U.S. patent application of K. H. Mueller (Ser. No. 636,297, filed Nov. 28, 1975) an echo cancelling signal is synthesized in a transversal structure having taps spaced at baud intervals, rather than at Nyquist intervals, from samples of the baseband data to be transmitted prior to modulation or application to the hybrid network under the control of an error signal obtained at the output of the receiver portion of a data transceiver.
It is an object of this invention to provide improved simultaneous full duplex digital data transmission over two-wire communications facilities with full bandwidth utilization for each transmission direction.
It is another object of this invention to provide an improved echo cancellation signal for full duplex data transmission systems from baud-rate samples of the transmitted signal.
It is a further object of this invention to provide jointly for echo cancellation and forward equalization responsive to a common digital error signal in a simultaneous full duplex, full bandwidth digital data transmission system.
It is yet another object of this invention to provide jointly for echo cancellation and forward equalization in a manner which compensates for differences in the sampling rates for transmitted and received digital data signals.